1. Field of the Invention
This invention relates to a lubricating oil composition. More particularly, the present invention relates to a lubricating oil composition which comprises a base oil for a lubricating oil and a specific molybdenum dithiocarbamate, has high stability over a long period of time, and has consistently excellent lubricating performance from the initial stages of use until even after its degradation.
2. Description of the Prior Art
In recent years, increasingly compact and maintenance-free lubricating system with higher performance have been required for lubricating systems. Also, required are a reduction in energy loss and an improvement in mechanical efficiency so as to meet recent energy saving trends.
Lubricating oil degrades over time due to exposure to physical shearing forces, high temperatures, high pressures and an oxidizing atmosphere during use. Though a portion of the additives added to the lubricating oil exhibits extreme pressure performance as in the case of an extreme pressure agent, the major proportion of additives oxidizes and degrades under the conditions described above, thereby decomposing and in some cases even changing into sludge, before exhibiting extreme pressure performance.
In internal combustion engines, in particular, blow-by gases such as NO.sub.x, SO.sub.x, hydrocarbons, and so forth, mix with the base oil, creating even harsher degradation conditions.
Attempts have been made in recent years to improve the temperature/viscosity characteristics of the base oil itself in order to improve the problems of wear and seizure at high temperatures and the problem of the energy loss at low temperatures. Refined mineral oils and synthetic oils prepared by chemical means have been used as the base oil, but the problem of decreased thermal stability of the base oil for a lubricating oil develops because those impurities contained in the mineral oils which have oxidation prevention functions such as sulfur compounds, nitrogen compounds, etc. are eliminated. A still greater problem is that base oils for lubricating oils which comprise a mineral oil and/or a synthetic oil, the viscosity index of which is at least 115 and the viscosity of which is within the range of 2 to 50 cSt at 100.degree. C. (hereinafter referred to as the "high VI Oil") have a high paraffin content and for this reason, solubility of additives in the lubricating oil is likely to decrease.
Molybdenum dithiocarbamate (hereinafter referred to as "MoDTC") has been used as an excellent additive for improving wear and friction under such conditions. However, because MoDTC has low solution stability in the high VI oils and the synthetic oils described above and as it may also function as an antioxidant, lubricating oils using MoDTC have low oxidation stability and are likely to fail to exhibit their lubricating characteristics.
In consideration of the object of reducing energy loss, a composition which allows MoDTC to optionally exhibit its function as a friction regulator but not as an antioxidant is very important.
Lubricating oil compositions which accomplish savings in fuel costs during the initial stages have been developed, but studies on such compositions have been mainly directed to new oils which have not yet begin to degrade, and studies on the durability of the additives are still scarce- Because lubricating oils degrade over time with use as described above, the effect of savings in fuel cost cannot be sufficiently obtained unless the low friction and low wear properties are maintained over a long period of time.
Japanese Patent Laid-Open No.62-81396, for example, proposes a molybdenum-containing lubricant additive with an excellent oxidation preventive function, wear proofing function, friction mitigating function and metal corrosion inhibiting function, and further is highly soluble in a base oil such as a mineral oil. Japanese Patent Laid-Open No.48-56202 proposes an extreme pressure lubricant containing MoDTC blended thereto.
Further, Japanese Patent Laid-Open No.5-279686 proposes a lubricating oil composition for an internal combustion engine prepared by blending (a) sulfurized oxymolybdenum dithiocarbamate and/or sulfurized oxymolybdenum organophosphorodithioate, (b) fatty acid ester and/or organoamide compound, (c) at least one compound selected from the group consisting of calcium sulfonate, magnesium sulfonate, calcium phenate and magnesium phenate, (d) at least one compound selected from the group consisting of benzylamine and boron derivatives of benzylamine, and (e) zinc dithiophosphate and/or zinc dithiocarbamate, in a base oil for a lubricating oil.
Japanese Patent Laid-Open No.5-230485 proposes a lubricating oil composition for an engine oil containing, as essential components in a base oil using a mineral oil and/or a synthetic lubricating oil, (a) an alkaline earth metal salt of alkylsalicylic acid, (b) a his-type alkenylsuccinic acid imide having a polybutenyl group and/or its derivative, and (c) sulfurized oxymolybdenum organophosphorodithioate and/or molybdenum dithiocarbamate.
Japanese Patent Laid-Open No.5-186787 proposes a lubricating oil composition prepared by blending (a) sulfurized oxymolybdenum dithiocarbamate and/or sulfurized oxymolybdenum organophosphorodithioate and (b) zinc dithiophosphate and/or zinc dithiocarbamate into a mineral oil, and further proposes a lubricating oil composition prepared by adding (c) an organic amide compound to the above.
Japanese Patent Laid-Open No.5-163497 proposes an engine oil composition comprising (A) a base oil consisting of a mineral oil and/or a synthetic oil, (B) a boron compound derivative of alkenylsuccinic acid imide, (C) an alkaline earth metal salt of salicylic acid and (D) molybdenum dithiophosphate and/or molybdenum dithiocarbamate, as the principal components.
However, none of the prior art technologies described above have succeeded in solving the great problems of molybdenum compounds, particularly MoDTC. In other words, the problems of the solubility of the MoDTC itself in a high VI oil of its residuary properties after oxidation and degradation, and of the extreme pressure properties of the lubricating oil composition after degradation (wear resistance during high load), still remain unsolved. The problem of solubility in a high VI oil being a particularly great problem.